The invention relates to a process for the production of silver-tin master alloys for dental amalgams having improved stability and processibility.
As known, amalgams are extensively used as filling materials in dental care.
The preparation of the amalgam in dental practice takes place immediately before the filling is to be applied by mixing mercury with silver-containing master alloys.
These master alloys as a rule comprise the main components silver and tin to which minor pecentages of other metals, in particular, copper are added.
The composition of these alloys predominantly correspond to the formula Ag.sub.3 Sn. This composition is very brittle and produces fine crumbling chips when machined by milling or turning which chips can be optionally further reduced in size by subsequent grinding.
It is assumed that during amalgamation, part of this Ag.sub.3 Sn (.gamma.-phase) is converted to a silver-mercury compound (.gamma..sub.1 -phase) and a tin-mercury compound (.gamma..sub.2 -phase). The latter corresponds approximately to the composition Sn.sub.8 Hg and is responsible for the corrosion sensitivity of this type amalgam which results from its high tin content.
The corrosion sensitivity can be substantially reduced by alloying the silver-tin master alloy with about 10 to about 25% copper. In this silver-tin-copper master alloy copper-tin compounds such as Cu.sub.6 Sn.sub.5 or Cu.sub.3 Sn are formed instead of Sn.sub.8 Hg as a result of higher affinity of copper to tin. The amalgams produced from such master alloys are therefore then extensively free from the .gamma..sub.2 -phase and substantially more corrosion resistant than the conventional silver-tin amalgams with acceptable silver content.
However, alloying with copper has certain drawbacks and disadvantages in the producibility of the pulverized master alloys and the advantage of the copper addition becomes questionable.
In order to obtain a good tamping of the alloy, a high edge strength of the filling and a certain tolerance in mercury dosage, pulverized alloys produced by machining are used predominantly. However, silver-tin-copper alloys can only be machined within a range of content of about 40% slver, 30% tin and 25 to 30% copper with the alloy having a ratio of about 3:2 of the brittle intermetallic compounds Ag.sub.8 Sn and Cu.sub.3 Sn. Amalgams produced from this alloy have as a result, a high copper content, they harden very slowly compared to the conventional amalgams and do not show the true silvery brilliance with the suggested zinc addition.
The currently commercially available products mostly comprise machined silver-tin alloys having a content of about 70% silver and 30% tin admixed with a pulverized silver-copper eutectic. Since this spherical pulverized powder must be finer than the chips, the yield from the pulverizing process is only about 25%. This pulverized powder is per se extremely corrosion sensitive and the powder mixtures produced in this way can only be stored under vacuum or absolute air exclusion and tend to separate during transport and processing. The amalgms produced from these powders are, moreover, uneconomical since their silver content is much higher than necessary.
With respect to desired composition, optimum master alloys for the preparation of amalgams are master alloys containing about 40 to 70% (preferably 50 to 60%) of silver, about 10 to 25% (preferably 12 to 15%) of copper and up to a maximum of 30% (preferably 20 to 30%) of tin and optionally other metals in minor amounts (for example, up to a maximum of 2% zinc). These alloys form long chips, however, as a result of their tenacity. Powders of these compositions can only be produced by pulverization with high pressure water or inert gas but powders pulverized in that way in general have the additional disadvantage that they can be poorly tamped. Their use requires a very precise mercury dosage and presupposes a very fine particle size. Also, the edge strength is poorer than in fillings produced from machined alloys.